There are tandem-type image forming apparatuses, in which toner images of yellow, magenta, cyan, and black (hereinafter referred to as Y, M, C, and K, respectively) colors are formed on image carrying members, e.g., photoconductors and subsequently superimposed and transferred onto either a belt member or a recording sheet on the belt member.
In the tandem-type image forming apparatus, owing to, for example, a change in temperature causing a slight change in position of an optical path of an optical system performing optical writing of latent images on the image carrying members, there can arise relative differences in latent image forming position among the four image carrying members for the Y, M, C, and K colors and resultant misregistration of the toner images of the respective colors. The misregistration of the toner images of the respective colors can also caused by a change in linear velocity of the four image carrying members and the belt member due to a variety of factors. The background image forming apparatus, therefore, performs a misregistration reduction process at a predetermined time, such as immediately after power-on and every time a predetermined number of sheets are printed. In the misregistration reduction process, respective position detection toner images formed on the image carrying members for the Y, M, C, and K colors are first transferred in a line onto the surface of the belt member to form misregistration detection patterns. Relative misregistration amounts of the position detection toner images of the respective colors are then detected on the basis of the timing of detection by reflective optical sensors of the position detection toner images of the respective colors in the misregistration detection patterns. Then, on the basis of the detection result, an image forming condition, such as the latent image writing start time, the face inclination angle of a reflecting mirror of the optical system, and the drive velocity change pattern of the image carrying members or the belt member, is adjusted to reduce the misregistration of the toner images of the respective colors.
The background image forming apparatus is configured to perform a solid density stabilization process in addition to the above-described misregistration reduction process. The solid density stabilization process is performed for the following reason. That is, in an electrophotographic image forming apparatus, if any of toners and a variety of members in the image forming apparatus undergoes a change in characteristics in accordance with a change in environment, such as temperature and humidity, there arises a change in toner adhesion amount per unit area of an output toner image and a resultant change in image density. Therefore, the solid density stabilization process is regularly performed to stabilize the image density of a solid image. In the solid density stabilization process, solid tone patterns each including a plurality of solid toner images different from one another in toner adhesion amount per unit area are first formed on the respective surfaces of the image carrying members for the Y, M, C, and K colors, and subsequently transferred in a line onto the surface of the belt member. In each of the solid tone patterns of the Y, M, C, and K colors transferred to the surface of the belt member, the toner adhesion amounts of the plurality of solid toner images are then detected by the reflective optical sensors. Then, one or more image forming conditions, such as the development potential and the intensity of latent image writing light, is adjusted on the basis of the detection results, to thereby form a solid image with a target image density.
Meanwhile, an image forming apparatus expressing a variety of tones by reproducing halftone in accordance with area coverage modulation has been widely used in recent years. In an image forming apparatus of this type, even if the image density of the solid image is stabilized by the solid density stabilization process, the image density of a halftone image according to area coverage modulation may be changed. This is because the image forming conditions, such as the development bias, for adjusting the toner adhesion amount per unit area of the solid image to a predetermined value are not necessarily the same as the image forming conditions for adjusting the toner adhesion amount per unit area of the halftone image according to area coverage modulation to a predetermined value.
In view of the above, another background image forming apparatus performs, separately from the solid density stabilization process, a halftone density stabilization process for stabilizing the image density of the halftone image according to area coverage modulation. The background image forming apparatus first performs the solid density stabilization process to adjust the development bias to a value providing a target image density. With the use of a reflective optical sensor, the background image forming apparatus then detects the toner adhesion amount of an area coverage modulation toner image of a halftone density formed under the adjusted development bias condition, and adjusts the image area ratio of the halftone image on the basis of the detection result to obtain a target halftone density. According to this configuration, the target image density is stably obtained in both the solid image and the halftone image. If the solid density stabilization process is performed after the halftone density stabilization process, the image density of the halftone image adjusted to the target image density deviates from the target image density owing to the change in, for example, the development bias. It is therefore desirable to perform the halftone density stabilization process after the solid density stabilization process, as in the above-described background image forming apparatus.
The above-described background image forming apparatus forms only single-color images. Also in a tandem-type image forming apparatus forming color images, it is desirable to perform the halftone density stabilization process for respective colors to improve the image quality. However, if the halftone density stabilization process starts after the completion of the solid density stabilization process, as in the above-described background image forming apparatus, the down-time of the image forming apparatus is substantially extended. After the start of the solid density stabilization process, therefore, it is desired to perform at least an early-stage step of the halftone density stabilization process concurrently with the solid density stabilization process, to thereby minimize an increase in down-time.
However, it has been found difficult to minimize the increase in down-time in the above-described manner. Specifically, the above-described area coverage modulation toner image is formed at an early stage of the halftone density stabilization process. To perform an early-stage step of the halftone density stabilization process concurrently with the solid density stabilization process, at least a latent image forming step of the area coverage modulation toner image should be performed concurrently with a step of the solid density stabilization process. As described above, however, it is desirable that the latent image of the area coverage modulation toner image be formed under the image forming condition adjusted by the solid density stabilization process. This configuration, therefore, allows the halftone density stabilization process to start only after the completion of the solid density stabilization process, and presents an obstacle to the minimization of the increase in down-time.